The invention relates to a resistance heated vaporizing apparatus for the vacuum depositing of thin coatings, especially of metal, onto vertically disposed objects of large surface area, such as sheets of glass or plastic.
Such vaporizers are used preferably for the uniform coating of large surfaces in large industrial vaporizing installations. In one known process for the vapor depositing of thin coatings onto sheets of glass or plastic in vacuo, which is used in the manufacture of heat reflecting glass panes, a plurality of vaporizing sources are disposed in a vertical plane in the form of a vaporizing field. The surfaces to be coated, such as large sheets of glass or plastic for example, are disposed in equidistant parallel planes on both sides of this vaporizing field.
As previously stated, large industrial vapor depositing installations of this kind are used primarily for the depositing of a heat reflecting metal coating from a vapor onto large glass sheets, in conjunction in some cases with adhesive and/or interference coatings. It is primarily a coating of gold that is deposited as the heat reflecting layer, such gold coated glasses being used for protection against sunlight both in the glazing of buildings and in the glazing of motor vehicles.
For the vacuum depositing of gold, tungsten coils can be used. A known vaporizing coil of this kind consists of a round tungsten rod with tungsten wire spirally wound thereon as described, for example, in B. L. Holland, "Vacuum Deposition of Thin Films," Chapman & Hall Ltd., London, 1963, pages 115 to 117. The gold in wire form is wound into the gaps between the turns of the vaporizing coil. When the vaporizing coil is heated by electrical resistance heating, the molten gold runs uniformly in the gaps so that the vaporization takes place uniformly on all sides. Accordingly, with vaporizing coils of this kind, gold coatings of equal thickness can be produced in a repeatable manner.
A disadvantage of the use of gold, especially in the vapor coating of large surfaces, consists in the high material costs. Appreciable savings of manufacturing costs can be achieved in this regard by replacing the gold coating with a silver coating, which has virtually the same protective action against sunlight. The advantages of replacing the gold coating with a silver coating have been described.
In the vaporization of silver by means of the vaporizing coils described above, the difficulty arises that tungsten is not wetted by silver. Instead, drops of silver depending from the turns of the tungsten coil are formed, and the vaporization characteristic is no longer uniform and reproducible. The use of other known vaporizer materials, such as molybdenum, tantalum or niobium, in the vaporizing coils brings no improvement in this respect. The use of tantalum and niobium does substantially prevent the formation of depending drops of silver, but even so the silver does not run uniformly in the interstices, so that vaporization towards both sides of the vaporization field is uneven and is not repeatable from batch to batch. On account of the variation of the vaporization characteristics from batch to batch, repeatable coating thicknesses cannot be achieved with vaporizing coils.
Other known vaporizing devices, such as shuttle vaporizers, are not suitable for the vapor coating of vertically disposed surfaces because the current of the vaporizing material from such devices flows upwardly. On the other hand, the horizontal arrangement of large sheets in big industrial vapor coating installations entails such great difficulties as to make it unfeasible.